Solid state lighting devices are well known in the art, and are rapidly replacing conventional lighting sources, such as incandescent bulbs and fluorescent lighting fixtures. As used herein, the term Light Emitting Device (LED) means a solid state lighting device, such as a light emitting diode or laser diode. The numerous advantages of LEDs over conventional lighting sources include: LEDs consume less energy to produce comparable lighting; LEDs do not generate heat on the scale of incandescent lights or even the ballasts of fluorescent fixtures; LEDs are generally fabricated with plastic or epoxy housing and lenses, and hence are more rugged; LEDs last longer than conventional light sources; LEDs do not contain toxic gases which may be released on breakage; and LEDs require no “warm-up” and may be cycled at high frequencies.
Due to these advantages, light fixtures, also known as luminaires, became commercially available soon after the development of practical white light LEDs. These luminaires typically assemble a homogenous plurality of white LEDs in series into a single string, driven by a power supply that provides a substantially constant drive current. Many such power supplies include a “dimming” or brightness control feature, whereby the constant output current can be adjusted, either in discrete steps, or continuously over a range, to drive the LED string at different levels of illumination (also referred to herein as luminous flux). As used herein, the term “constant current power supply” refers to a power supply circuit that attempts to output a substantially constant current to a load, at any particular selected drive (brightness) level.
Light sources—both individual LEDs and luminaires constructed with them—may be characterized by various luminous characteristics, such as Correlated Color Temperature (CCT), and luminous flux.
The chromaticity, or color, of a light source may be expressed as a “color point” in a coordinate system of a color space, such as a tristimulus value (X, Y, Z) or the color coordinates (CCx, CCy) on a chromaticity diagram. Alternatively, because a black-body radiator emits a range of colors depending on its temperature, color information may also be expressed as a CCT, which is the temperature (on the Kelvin scale) at which the heated black-body radiator matches the color of the light source. The CCT of white light sources ranges from around 2700 K to 6500 K. Light at the lower end, around 2700 K, has a yellowish color (referred to as “warm” white light), and light at the upper end, around 6500 K, has a blueish color (referred to as “cool” white light).
The luminous flux of a light source refers to the intensity of its illumination, measured in lumens, where one lumen is the amount of light emitted per second in a unit solid angle of one steradian from a uniform source of one candela. The luminous flux of most LEDs varies with the current with which it is driven; however, LEDs may be manufactured to different sizes and hence may have different luminous flux capabilities.
Early LED luminaires typically included a single string of homogeneous white light LEDs, all outputting the same luminous characteristics, and all driven together in series. For example, all LEDs in such a luminaire may be white Light Emitting Diodes. A white LED is structurally a blue LED that includes a phosphor which absorbs some of the high-energy blue light and emits lower-energy yellow, green, and red light. The phosphor-converted light mixes with an unchanged portion of the blue light, producing perceptually white light. The single-string luminaire has a set CCT, depending on the particular LEDs used, which is typically on the cool end of the spectrum.
More modern LED luminaires mix, or blend, the light from a plurality of heterogeneous strings of LEDs, in fixed or controllable proportions. For example, one string may comprise Blue-Shifted-Yellow (BSY) LEDs. Similarly to the white LEDs described above, BSY LEDs emit blue light, and phosphors shift some of the light to the yellow range of the spectrum; the combination of blue and yellow produces a cool white light. Another string may comprise red or red-orange (RDO) LEDs, which produce a warmer CCT of light.
The intensities of the strings, and hence the CCT of blended light output by the luminaire, may be independently controlled by altering the current supplied to each string. The ratio of intensities may be predetermined, user programmed, or dynamically changed in response to various inputs or sensed conditions. The cost of the luminaire product generally scales according to these options. That is, a fixed (predetermined) CCT luminaire is generally the least expensive option; user control adds cost, as it often requires multiple-channel drivers; and the “smart” luminaires that alter their lighting automatically are the most expensive. The same maxim is true for fixed/controllable/automatic luminosity. In many industrial, commercial, and office environments, the lower cost option—predetermined CCT and luminosity—comprises the bulk of the lighting market. However, different applications, environmental conditions, customer preferences, and the like lead to a large number of permutations of different predetermined values of CCT and/or luminosity. Distributors must thus maintain a large stock of luminaire products, the only difference among which are the predetermined values of CCT and luminosity, both of which are controlled by the level and ratio of drive current supplied to heterogeneous strings of LEDs.
The Background section of this document is provided to place embodiments of the present invention in technological and operational context, to assist those of skill in the art in understanding their scope and utility. Unless explicitly identified as such, no statement herein is admitted to be prior art merely by its inclusion in the Background section.